This invention relates to steam turbine rotors, and more particularly it is concerned with a novel steam turbine rotor shaft made of high chromium and high nickel austenite steel having at the journal section thereof a plurality of layers of build-up welding that have high strength and excellent bearing characteristics.
To cope with the situation of shortage of oil and its high prices, a generating station has a tendency to be operated at high temperature under high pressure so as to improve the operation efficiency thereof. Nowadays steam turbine rotors operate under the steam condition of 538.degree. C., and Cr-Mo-V steel and 12% Cr base steel constitute the main body of the steam turbine rotors. However, these materials can not be used under the high temperature condition of over 600.degree. C. and high pressure because they are insufficient for high temperature strength and are low in resistance to oxidation at such high temperature.
Meanwhile high Cr and high Ni austenite stainless steel has an excellent high temperature creep rupture characteristic and is highly resistant to oxidation at high temperature, so that this stainless steel is suitable for use as materials for fabricating steam turbine rotors capable of operating under high temperature and pressure.
FIG. 1 of the drawings shows a conventional improved construction of a journal section of a rotor shaft made of 12% Cr steel. The rotor shaft has at one end portion of a rotor 1 a coupling 3 fitted thereon by shrink fit for coupling the rotor 1 to a shaft 2 of a generator. However, it is considered best that the coupling be formed integrally with the rotor shaft.
Japanese Patent Laid-Open Publication No. 16744/80 discloses a construction of the journal section of a rotor shaft in which, in order to improve such inferior bearing characteristic, there is provided a layer 5 formed by build-up welding by use of an electrode containing about 0.1% Cr.
The inventors have found that the high Cr and high Ni austenite stainless steel used for producing the rotor shaft capable of being operated at higher temperature is inferior in bearing characteristics with such a drawback being caused that a bearing 4 is apt to be injured at a journal portion 5 made of the high Cr and high Ni stainless steel.
The inventors have also found that it is impossible to improve the bearing characteristics of rotor shafts made of high Cr and high Ni austenite steel even if there is merely effected such build-up welding as described in the prior art with regard to the conventional rotor shaft made of 12% Cr steel. When build-up welding is effected, it is necessary to subject the rotor shaft to heat treatment at a high temperature of about 1000.degree. C. to reduce residual stress in the heat-affected zone of the austenite steel and render uniform the structure and hardness. In the example described hereinabove of effecting build-up welding by using an electrode containing about 0.1% Cr, however, a reduction in strength of the build-up welding portion would result if the rotor shaft is subjected to heat treatment at such high temperature. If welding rod of low Cr content is used for effecting build-up welding in a rotor shaft made of high Cr steel and then the rotor shaft is subjected to heat treatment at a high temperature of about 1000.degree. C., the ductility of deposited metal would be inordinately reduced in the first and second layers thereof and cracks are apt to be caused therein while in service, thereby causing rupture of the rotor in the worst cases.
The object of the present invention is to obtain a steam turbine rotor shaft made of precipitation-hardened austenite steel in which shaft there is provided a journal section superior in bearing characteristics.
According to the invention, there is provided a steam turbine rotor shaft formed of forged austenite steel that is subjected to precipitation hardening, wherein the rotor shaft comprises a metal member having bearing characteristics superior to those of such forged austenite steel, which metal member is provided on an outer surface of a bearing portion in a journal section thereof.
The outer surface part in the bearing portion of the journal section of the rotor shaft is preferably formed of a material containing Fe as its principal constituent and Cr in 0.3-3.0 wt %. Particularly, the metal forming the outer surface part of the bearing portion preferably consists essentially of not more than 0.1% C, not more than 1.0% Si, not more than 2.0% Mn, 0.3-3.0% Cr, not more than 2.0% Mo and the balance substantially Fe. More specifically, the metal preferably consists essentially of 0.04-0.08% C, 0.1-0.5 Si, 0.3-1.0% Mn, 0.5-1.0% Cr, 0.3-1.0% Mo and the balance substantially Fe.
When the metal member is provided by build-up welding, a surface portion in the layers of build-up welding may preferably be of tempered martensite structure predominantly while having carbides precipitated therein.
The austenite steel of high Cr and high Ni contents forming a main body of the steam turbine rotor shaft according to the present invention has a .differential.' phase and consists essentially, by weight, of 0.01-0.1% C, not more than 1.0% Si, not more than 2.0% Mn, 12-16% Cr, 24-27% Ni, 0.5-1.5% Mo, 1.0-2.5% Ti, 0.2-0.4% Al, 0.1-0.4% V and the balance substantially Fe. More preferably, the austenite steel further includes 0.003-0.008% B by weight. Preferably the steel according to the invention may be softened by quenching after being held at 900-1100.degree. C. and then may be precipitation-hardened because of Ni compounds and carbides by being slowly cooled after being held at a temperature in the range between 650.degree. and 750.degree. C.
Before effecting welding, it is necessary to soften the steel according to the present invention by subjecting the steel to heat treatment in which the steel is quenched after being held at a temperature in the range between 900.degree. and 1100.degree. C. If welding is performed directly after precipitation hardening has been effected, there is a possibility that weld crack might occur due to embrittlement of the welding heat-affected zone.